The present invention is a method for obtaining an accurate temperature measurement of a hot mineral material. The method is particularly useful for measuring the temperature of products such as lime or cement clinker within or as they are discharged from a rotary kiln.
One key piece of information necessary for the efficient operation of a rotary kiln is the temperature of the product as it exits the kiln. A lime kiln may be considered as exemplary. If the product exit temperture is too high, then the lime is being over-burned. One characteristic of such a product is that it forms into nodules having hard shells. If used in a chemical process, this product will be less reactive than desired. A further, and perhaps even more undersirable problem with overburning is the unnecessarily excessive use of fuel. On the other hand, if the product temperature is too low, the calcium carbonate may not be fully converted to the oxide form. Once again this results in reduced reactivity and, in chemical processes such as kraft pulping liquor preparation, can result in increased deadload being carried around the cycle. A further problem with under-burning is poor nodulization of the lime. This can cause severe dusting conditions in the kiln with sizable fractions of the converted lime being entrained in the flue gases. Frequently this results in precipitator overload and significant loss of lime from the stack. This condition also wastes energy and kiln capacity since the material lost as dust has undergone the normal conversion process in the kiln. Similar problems occur during manufacture of the various types of Portland cement.
On first consideration, it would seem that accurate temperature measurement of the product within or being discharged from a kiln would be a very simple problem. Unfortunately, this is not the case. The two methods normally used; i.e., thermocouples and infrared pyrometers, are both inappropriate in this environment.
Thermocouples tend to have extremely short life spans because of the high temperature environment, high chemical reactivity and abrasive character of the product, and the extreme mechanical forces imposed by occasional very large nodules. In addition, unless the thermocouple is completely immersed in the product, substantial correction for radiative heat transfer is required. Unfortunately, the magnitude and even the sign of this correction cannot be readily or accurately determined.
Optical pyrometers are in some ways even more unsatisfactory. The optical properties of lime are such that it is highly transparent to radiation of all wavelengths from the mid-ultraviolet to the mid-infrared. Therefore, in the form that exists in a kiln, lime is a very good scatterer of radiation in the range from the near ultraviolet, through the visible, and well out into the infrared and does not exhibit any appreciable absorption. It follows from this that since the absorptivity is very low the emissivity is also very low. It is likely that the temperature indicated by a pyrometer viewing the lime surface will be heavily influenced by reflected radiation which originates at the kiln wall or in the flame. In the infrared, at wavelengths longer than 15 um where lime does show significant absorption, the absorption of radiation by water vapor and carbon dioxide will produce an additional substantial interference. Actual temperature measurements of the lime itself, when obtained by pyrometer, are likely to be substantially in error. This is especially unfortunate since the advantages of optical measurement are considerable. They can be nonintrusive, continuous, and can be made on-line without the need for physically sampling the material being measured.
In recent years a number of laser-based measurement techniques have been developed for measuring temperature, composition, velocities and other characteristics of matter in extremely harsh environments. In these techniques, the interaction of the laser radiation with the material under study is examined to determined some property of the material. Laser-induced fluorescence has been employed for temperature measurement although it has never been suggested for use in the hostile environment of an industrial kiln operation. In fact, a number of significant potential impediments immediately come to mind with regard to the use of this technique for kiln product measurement. To better understand these impediments, and how they have been overcome in the present invention, it is necessary to briefly describe the use of laser-induced fluorescence for temperature measurement.
Fluorescence is a property of certain materials whose electrons cna readily undergo transitions between higher and lower energy states. The higher energy state is usually brought about by excitation by light in a given spectral range, by x-rays, or by electron beam irradiation. When the source of excitation is removed, the electrons return to their lower energy state and emit light having a spectral distribution characteristic of the particular material. Phosphors, as these fluorescent materials are called, are commonly used as a coating on the screen portion of cathode ray tubes and on the interior surface of fluorescent lights, as well as for many other purposes. Without phosphors having various spectral emission characteristics color television, or for that matter black-and-white television in its current form, would be impossible.
Phospors are usually crystalline, inorganic materials which are "doped" with small amounts of an impurity which enters the crystal lattice. In most cases the host material itself shows little or no fluorescence or a fluorescence which is in an undesirable spectral range. Over the last two decades rare earth dopants have come into wide use in the preparation of phosphors. There are a number of reasons for this. Rare earths produce highly efficient phosphors--ones which fluoresce strongly. Also, the fluorescence spectra of rare earth doped phosphors generally exhibit emissions in discrete narrow bands. Selected phosphors; e.g., those used in color television tubes, show brilliant fluorescence as reds, greens and blues in the visible light range. Other fluorescence bands may be completely outside the visible light range, either in the infrared or ultraviolet.
The general knowledge that rare earth doped phosphors can be useful in instrumentation is well reported in the literature. Wickersheim et al, "Study of Rare Earth Activated Materials for Radiation-sensing Applications," 3rd Annual Report, Lockheed Missiles & Space Company, U.S. Atomic Energy Commission, Division of Biology and Medicine, Contract No. AT(04-3)-674 (1969), report the usefulness of rare earth doped phosphors in scintillation counters, x-ray conversion and image intensifier use, neutron detection, and radiation dosimetry. Hughes and Pells, J. Phys. C.: Solid State Phys. 7: 3997-4006 (1974), show that gadolinium doped calcium oxide spectra show a temperature dependency between 6.degree. and 300.degree.K. Kolodner and Tyson, Appl. Phys. Lett. 42(1): 117-119 (1983), teach that a thin fluorescent film may be coated on a thin conductive film which is subsequently heated by an electric current. Localized temperatures are determined to a high degree of accuracy using apparatus equivalent to a fluorescence microscope. The technique is said to be particularly useful for looking at surface temperature profiles on integrated circuit chips. Another article in the same field by Wickersheim and Sun, Research and Development, 27(11): 114-119 (1985), describes problems of surface temperature measurement in integrated circuit manufacturing using an optical fiber system.
Japanese Kokai No. 60[1985]-250640 teaches temperature measurement of a localized area on an integrated circuit chip surface by focusing a collimated laser beam to a small point and analyzing fluorescence.
Alves et al, Advances in instrumentation 38(2): 925-932 [Proc. Inst. Soc. Am. Int'l. Conf., Oct. 10-13, 1983, Houston, Tex.](1983), describe fluorescence thermometry using ultraviolet light activated phosphors with an optical fiber system.
Hirschfeld, in U.S. Pat. No. 4,542,987, describes temperature measurement using a fluorescent solid attached to one end of an optical fiber. A light source and fluorescence measuring equipment are attached to the other end of the fiber. In this case the fluorescent solid must be a single crystal or piece of a doped glass. British Patent No. 2,113,837 shows an optic fiber measuring device in which a fluorescent surface is excited by a pulsed radiation source and the decay time is measured as an indicator of temperature.
Wickersheim, J. Microwave Power & Electromag. Energy, 21:105-109 (1986), describes commercially available equipment for fluorescence thermometry. Two techniques are used. In one, the intensity ratio of fluorescence peaks at two different wavelengths is found to be dependent upon temperature. An alternate system uses fluorescence decay time after the phosphor is activated by a xenon flash lamp.
British Patent No. 2,064,107 describes the measurement of "a physical property" on which the fluorescent properties of a phosphor are dependent by measuring the fluorescent lifetime after being excited by a pulsed or modulated light source. Temperature is one physcial property that can be determined. The inventor describes a phosphor as being any compound which emits fluorescent radiation as a result of being irradiated with a different (shorter) wavelength energy source.
Cates et al, Laser Inst. Amer., Int'l. Conf. on Applic. of Lasers and Electro-Optics 49, 50, 51: 142-147 (1985), show the decay time versus temperature of a europium doped yttrium oxide phosphor over a temperature range of 300.degree.-1000.degree. C. The authors extrapolate their data and predict a usefulness of their system to 1400.degree. C. with this phosphor. The phosphor is suggested for use as a refractory coating on rotating turbine components in order to measure surface temperatures under operating conditions.
Canadian Patent No. 1,019,978 teaches the measurement of surface temperatures of moving bodies; e.g., turbine rotors. A phosphor strip is applied to the rotating body. The activator source is preferably ultravoilet light with the emitted light being in the visible range. Fiber optics are preferably used for light transmission.
Khare and Ranade, Indiam Jour. Pure & Appl. Phys. 13: 664-666 (1975), show that calcium oxide forms a phosphor using cerium and terbium activators by firing mixtures of the activators with lime at 1100.degree. C. The same authors in Jour. of Materials Sci. 15: 1868-1869 (1980) describe the use of calcium oxide as a host lattice for phosphors sensitized by cerium or gadolinium. These were prepared by firing calcium carbonate and the desired amount of a cerium or gadolinium compound for about four hours at 1000.degree. C. Dopant was used at a dosage of about 0.4 mole percent based on lime. Porter et al, Applied Spectroscopy 37(4): 360-371 (1983), show calcium oxide to be a good matrix for rare earth phosphors, which have little interference for lanthanide analyses. Porter and Wright, J. Chem. Phys. 77(5): 2322-2329 (1982), teach the preparation of doped calcium oxide materials prepared from extremely pure calcium carbonate and dopant by sintering at 1150.degree. C. for 6-48 hours.
Along the lines of more conventional kiln temperature mesurement, Bartran and Nelson, Chem. Eng. 92(13): 65-66 (1985), teach the modification of a kiln by the insertion of radial tubes through the kiln walls to provide wells for conventional thermocouples.
Two United States patents may be taken as representative of prior methods which attempted to measure the temperature of contents within a kiln. Drewry, in U.S. Pat. No. 3,647,195, describes the use of two internally located optical pyrometers whose output is transmitted through slip rings mounted around the outside of the kiln shell. Jager et al. in U.S. Pat. No. 4,487,575 also teach the use of infrared radiation for measurement of internal kiln conditions.
While laser-activated fluorescence is known for temperature measurement under certain conditions, it has not been heretofore used or suggested as a method for measuring the temperature of the contents in a heated kiln. There are a number of reasons why this potential use has apparently not been seriously considered heretofore. Natural minerals such as lime, which might require calcining in their processing, all contain high levels of impurities which could possibly interfere with measurements. This is in contrast to virtually all of the work just noted and to commercial rare earth phosphors which use extremely high purity host materials and rare earth dopants. Another possible reason is a perceived high cost of the rare earth dopant unless only trace quantities or crude forms would be suitable. Further, it was entirely unclear whether the residence time and temperature in a kiln would be sufficient to incorporate the rare earth into the crystal lattice of the host mineral so that a phosphor would even be formed. Also, in addition to the possible interfering spectra previously noted, some impurities are known to partially quench the fluorescence of rare earth phosphors.
Despite all of the above negative implications the present inventor has surprisingly found that calcareous and other types of minerals containing impurities can be economically doped with rare earths and that the use of fluorescence thermometry is a simple and accurate means of measuring a hot product temperature within or as it is discharged form a kiln.